It's a familiar expression, and may be perfectly acceptable in some professional settings. It would depend upon how you said it and under what context.
Your name indicates that you may speak French so I'll take a chance and offer another explanation.
Hang on = attends. If you wish to say the English equivalent of "attendez" and the situation does not permit you to "tutoyer", find another expression. "Please wait" is one possibility.
Student or Learner